In order to correct focusing error due to vertical motion of a disk-type recording medium (hereinafter referred to simply as "disk") such as a compact disk, tracking error due to decentration, or a tilt of an objective lens relative to the disk, an objective lens driver drives the objective lens in two axes, i.e., the direction perpendicular to the disk (hereinafter referred to as "focusing direction") and the radial direction of the disk (hereinafter referred to as "tracking direction"). It is to be noted that the focusing direction is the direction perpendicular to the disk and the tracking direction is the radial direction of the disk on the premise that the disk loaded on the driver is in ideal shape having no curvature and no decentration.
In an optical recording/reproduction apparatus including such an objective lens driver, in order to increase the recording capacity, recording and reproduction are carried out with a converging spot of reduced diameter using an objective lens of high numerical aperture. In this case, since the degree of aberration attendant on a tilt of the objective lens relative to the disk increases in proportion to third power of the numerical aperture, positioning of the optical axis of the objective lens relative to the disk must be precisely performed to obtain satisfactory recording/reproduction signals.
For this purpose, the conventional optical recording/reproduction apparatus is equipped with a tilt correction controller for correcting a tilt by inclining the whole optical head using a DC motor or the like.
However, the tilt correction controller has the following drawbacks. Since this means can correct only angle errors of low frequencies, it is difficult to reduce aberration. In addition, since a mechanism for inclining the whole optical head is added, the size of the optical recording/reproduction apparatus is increased. Further, since it is very difficult to match the rotation center at the tilt correction with the principal point of the objective lens because of lack of space, the height of the objective lens significantly varies with the rotation and, therefore, some means for adjusting the height is needed, resulting in a further increase in the size of the apparatus. It is to be noted that the principal point of the objective lens is a point on the optical axis of the objective lens (shown by 100 in FIG. 9(b)), and a distance between this point and the focal point is the focal length.
Meanwhile, Japanese Published Patent Application No. Hei. 4-366429 discloses an objective lens driver that detects a relative tilt of the disk and the optical axis of the objective lens and rapidly corrects the tilt.
Hereinafter, the objective lens driver disclosed will be described in detail.
This objective lens driver is equipped with an objective lens for focusing a light beam on an optical information recording medium; a holder for holding the objective lens; a supporter for supporting the holder movably in the focusing direction and tiltably to the focusing direction; a plurality of driving coils for moving the objective lens in the focusing direction and tilting the objective lens to the focusing direction by driving the holder in response to voltages applied thereto, which coils are disposed on the holder individually and symmetrically about the center line of the objective lens; and a magnet for generating a force for driving the holder between itself and the driving coils, which magnet is fixed onto the supporter.
The objective lens driver is so constructed operates as follows. It is assumed that the objective lens driver includes four driving coils, and the four driving coils are arranged, two by two, symmetrically about a plane including the center line of the objective lens and being perpendicular to the tracings direction. In this case, when the respective driving coils are supplied with currents of the same value in the same direction, the objective lens moves in the focusing direction without tilting to the focusing direction, whereby focusing control is carried out.
When an arbitrary current is applied to two of the four driving coils performing focusing control in response to the currents of the same value in the same direction, which two coils are disposed on either side of the plane including the center line of the objective lens and being perpendicular to the tracking direction, a portion of the of the holder having the driving coils supplied with the arbitrary current moves up or down in response to the current, i.e., the holder tilts to the focusing direction. Since the objective lens can be tilted to the focusing direction as described above, it is possible to correct a tilt of the optical axis of the objective lens to the disk.
Accordingly, a mechanism and a motor for tilting the whole optical head are dispensed with, resulting in a small-sized optical recording/reproduction apparatus capable of high-speed tilt correction.
In the objective lens driver described above, however, since the magnetic circuit for generating the operating force of the driving coils, i.e., the magnet and the supporter, is fixed, the current driving sensitivity in the tilt correction varies with the motion of thee holder in the focusing direction or the tracking direction. The reason is as follows. Usually, the magnetic flux density distribution in a void of the magnetic circuit has a shape like a mountain and, therefore, the amount of the magnetic flux interlinked with each driving coil varies when the position of the driving coil relative to the magnetic circuit varies.
Accordingly, the driving sensitivity in the tilt correcting direction varies according to the position of the holder, so that the stability of tilt correction control is degraded.
Furthermore, the rotation center in the tilt correction varies with the holder's moving in the focusing direction or the tracking direction. The reason is as follows. Since the magnetic flux density distribution in a magnetic void part of the magnetic circuit has a shape like a mountain as described above and, therefore, the relative position of the holder and the point of action of the driving force varies when the position of the driving coil relative to the magnetic circuit varies.
Since the rotation center in the tilt correction varies according to the position of the holder, the position of the objective lens in the focusing direction or the tracking direction varies due to the tilt correction, and the amount of the variation varies according to the position of the holder. Thereby, the control precision is degraded in focusing or tracking control.
Furthermore, when the holder moves in the focusing direction or the tracking direction, i.e., when the relative position of the holder and the point of action of the driving force varies, the point of action of the driving force in the focusing direction or the tracking direction deviates from the center of gravity of the holder, so that tilting of the holder may occur outside of the tilt correction.
Consequently, since the tilt angle at tilt correction varies according to the position of the holder, the control precision is degraded in controlling the tilt correction.